MY OPINION is the bench grinder is the worst
An angle grinder if you know how to use it properly the best at home.
I personally take mine to the shopand have then sharpened only 10 bucks for three blades is not a bad price.

How often? As often as needed to keep from tearing the grass.
I do mine weekly.

i bought the bradley blade sharpener and it doesn't work very well for commercial blades that are sharpened almost to the middle. I went back to my angle grinder with the 40 grit flap disc. it gets them much sharper and doesn't get them nearly as hot as a normal grinder disc does.

Thanks all for your replies, seems everyone has different opinions on this subject. Every method has pros and cons depending on your blade design. The angle grinder seems the fastest but very inconsistent as far a nice straight edge. I even seen blades sharpened right on the mower. Iam sure those blades are not balanced.

Those Bradley's seems nice if your blades are straight and flat. Mulching blades are far from that.